Environmental Engineering Reference
In-Depth Information
inoculum production through open pot cultures
involves the following steps.
cots less dependent on mycorrhizal fungi than
dicots.
5.1.1.1 Isolation of AM Fungal Strain
Spores of desired AM fungal strain are collected
from the plant's rhizosphere by wet-sieving and
decanting technique (Gerdemann and Nicolson
1963
). However, for raising monoculture, AM
fungal propagules can be acquired from trap
cultures. The trap culture constitutes the host
plant rhizospheric soil diluted equally with steril-
ized sand (Menge
1984
). The resulting AM fun-
gal spore densities are higher compared to the
initial rhizosphere soil. In fact, undetected spores
in the initial extraction of fi eld soil could be
detected in trap culture (Mortan et al.
1995
). Liu
and Wang (
2003
) evaluated the presence of AM
fungal spores by using different trap plants. The
results of this study suggested that the numbers
of AM fungal spores and species were higher in
trap cultures and clover (
Trifolium repens
) was
found to be a suitable host for identifying the AM
fungal diversity. Using spores for initiating
cultures certainly has some benefi ts like easy
detection of undesired fungal spores, quick enu-
meration and evaluation of spore viability and
germination and a reduction of pathogen inclu-
sion (Dodd and Thomson
1994
). On the other
side, AM fungal spores sometimes exhibit dor-
mancy, which reduces the potential of the inocu-
lum (Gemma and Koske
1988
).
5.1.1.3 Optimizing Growing Conditions
Properly sterilized substrate is essential not only
to maintain the purity of a culture but also for
avoidance of diseases. Usually, equal propor-
tions of sterilized sand soil mixture are used for
raising inoculum (1:1; sand/soil). A coarse-tex-
tured sandy soil (Gaur and Adholeya
2000
)
mixed with vermiculite or perlite or turface
(Dehne and Backhaus
1986
) can also be used.
Inadequate mineral nutrient in the substrate may
affect plant and in turn the fungal development.
However, the excess of available P can inhibit
AM fungal propagation. As N, K, Mg and
microelement ratios may affect AM inoculum
development, plant fertilization needs to be per-
formed artifi cially especially when inert sub-
strates are used for inoculum production (Dixon
et al.
1999
). In addition, other edaphic and cli-
matic factors such as pH, soil temperature and
aeration, light intensity and relative humidity
need to be controlled for optimal AM fungal
propagation (Rao and Tarafdar
1999
). Some dis-
advantages of open pot-culture production
include bulkiness, transportation problems,
cross-contamination and lack of genetic stabil-
ity (Abdul-Khaliq et al.
2001
).
5.1.2 In Vitro Propagation on Root-
Organ Culture
The root-organ culture involves the proliferation
of excised roots under axenic conditions on an
artifi cial nutrient media supplemented with vita-
mins, minerals and carbohydrates. This method
was fi rst used for in vitro AM fungal propagation
by Mosse and Hepper (
1975
). Root-organ cul-
tures with vigorous root formation and uniform
growth under poor nutrient conditions (alteration
in hormones) have been obtained through the
transformation of roots by the soil bacterium
Agrobacterium rhizogenes
(Abdul-Khaliq et al.
2001
). Nevertheless, when hairy root technique
started to emerge, AM fungal propagules like
spores or sporocarps, mycorrhizal root bits and
even isolated vesicles were used in hairy root cul-
tures to initiate in vitro AM fungal inoculum
5.1.1.2 Choice of a Host Plant
The selection of a suitable host plant should be
based on high mycorrhizal dependency, adapt-
ability to in vitro or greenhouse conditions and
in having an extensive root system. The host
plants commonly used for raising pot cultures
are corn, onion, leek (
Allium porrum
),
Sorghum
halepense
, Bahia grass (
Paspalum notatum
),
Guinea grass
(
Panicum maximum
)
,
buffell
grass
(
Cenchrus ciliaris
)
and subterranean
clover
(
Trifolium subterraneum
) (Chellappan
et al.
2001
). Generally, monocots are preferred
as hosts, because the fi brous root system
enables uniform spreading of the roots in a
given volume of soil than plants with tap roots.
Further, the fi brous root system renders mono-
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